Probe thermostat



Oct. 14, 1958 H. w. BLETz 2,856,489

PROBE THERMOSTAT Original Filed Aug. 30, 1956 INVENTOR.

United States Patent O PROBE THERMOSTAT Howard W. Bletz, Lexington,Ohio, assignor to Stevens Manufacturing Company, Inc., a corporation ofOhio Original application August 30, 1956, Serial No. 607,110. Dividedand this application March 17, 1958, Serial No. 721,890

7 Claims. (Cl. ZOO-136.5)

This application is a division of my application Serial No. 607,110,filed August 30, 1956; entitled Probe Thermostat.

The prior art has shown long probe-like elements as a part ofthermostatic switch devices but these have proved unsatisfactory in usein either life tests or reliability tests. One prior art form of probetype thermostatic switch device incorporated an expansible tube withinwhich was contained a low expansion member welded at the outboard end tothe outer expansion tube and the inner end pulled through a linkage on aswitch contact to open the electrical switch. This prior art form thusrelied upon the satisfactory performance of three different welds and,if any of these welds pulled loose, the device would not open theelectrical circuit; and thus the electrical heating device supposedlyunder control would not be turned off but would continue to heat to itsmaximum capability thus creating a safety and fire hazard.

This prior art form of thermostat also was very unreliable incalibration for repeated rotation of the adjusting screw as this changedthe calibration markedly during the alleged useful life of thethermostat.

An object of the invention therefore is to provide a probe typethermostatic device which has long life and good reliability and whichremains correctly calibrated throughout its life.

Another object of the invention is to provide a probe type thermostaticswitch which forms a part of a connecting plug interconnecting a heatingelement of an electrical heating appliance and an energizing line cord.

Another object of the invention is to provide a probe type thermostaticswitch which has a high expansion member of aluminum which is of heavywall thickness and wherein an internal low expansion member is avitreous product wherein the combination of these two members havingdifferent thermal co-eicients of expansion result in a thermostat withmore thermal range and less critical adjustment of the entirethermostat.

Another object of the invention is to eliminate any welded linkage whichwould allow the contacts to remain in the closed position if any of thewelds Ifailed.

Another object of the invention is to provide a probe type thermostaticswitch in which any breakage of the low expansion member would cause thethermostatic switch to fail in the open contact position or any fractureof this low expansion member due to abuse would cause a slight drop inthe temperature calibration or cause the thermostat to fail in the opencircuit condition.

Another object of the invention is to provide a thermostatic probe as apart of an electrical connector having a plastic housing wherein thethermostatic unit is complete in itself and does not depend upon theplastic housing to maintain the calibration of the thermostat.

Another object of the invention is to provide a thermostatic switchwhich may be pre-calibrated at the point of manufacture regardless ofthe design of the heating appliance with which the thermostat may beused.

Another object of the invention is to provide a probe 2,856,489 PatentedOct. 14, 1958 ice type thermostat having an adjusting screw whereinrepeated adjustments of this adjusting screw do not materially affectthe temperature calibration for various settings of the adjusting screw.

Another object of the invention is to provide a thermostatic switchwherein contacts are mounted on spring blades of such material andconstruction as to maintain contact pressure and operating forces athigh temperature of the probe.

Another object of the invention is to provide a probe type thermostaticswitch Which may be adjusted to and heated to its maximum temperatureposition and then adjusted to its low temperature position and back tothe high temperature position and continuing this cycling indefinitelyand yet maintaining an off position of the contacts of the switchwhenever adjusted to the low temperature position.

Other objects and a fuller understanding of this invention may be had byreferring to the following description and claims, taken in conjunctionwith the accompanying drawing, in which:

Figure l is an elevational view partly in section of a probe thermostataccording to the invention connected to an electrical heating applianceillustrated as an electrical frying pan;

Figure 2 is a plan view of the probe thermostat;

Figure 3 is a longitudinal sectional view of the thermostat of Figure 2;

Figure 4 is a plan view of the probe thermostat removed frorn itsinsulating housing;

Figure 5 is an end view of the thermostat of Figure 4;

Figure 6 is a plan view of a contact blade removed from the thermostat;and

Figure 7 is a schematic circuit diagram of a circuit in which the probethermostat may be used.

The Figures 2 and 3 show the construction of the entire probe thermostat11 which includes a probe 12 and a control housing 13. The controlhousing 13 also includes an electrical plug including first and secondfemale connectors 15 and 16. This probe thermostat 11 may be used inconnection with an electrical heating appliance 18 which in this casehas been shown 'for purposes of illustration as an electrical fryingpan. This frying pan has a base 19 containing an elongated cylindricalaperture 20 and also containing a sealed-in heating element 21 shownschematically in Figure 7 and shown in Figure l as contained within araised boss 22. .The heating appliance 18 has first and second externalmale prongs 24 and 25 shown diagrammatically in Figure 7 to be receivedin the female connectors 15 and 16 when the probe 12 is received withinthe cylindrical aperture 20. This probe 12 is of a size and shape to beclosely received within this aperture 20.

The control housing 13 includes upper and lower halves 28 and 29connected together in any convenient manner, not shown, and generallymay be made of some insulating material such as plastic or at least theexterior surface of this control housing 13 is insulated from theelectrical switch mechanism 30 within the probe thermostat 11.

This switch mechanism 30, as best shown in Figure 3,'

includes a rigid frame 32 which may be of metal and is generallyL-shaped with long and short legs 33 and 34, respectively. A stack 35 iscarried on the long leg of the frame near the junction of the long andAshort legs 33 and 34. This stack includes first and second springcontact blades of metal 36 and 37 with first ends thereof mounted in thestack 35. The stack 35 is fastened together by a rivet 38 and the blades36 and 37 are mutually insulated and insulated from the frame 32 byinsulating washers 39. Terminals 40 and 41 are also mounted in the stack35 and are in electrical contact, respectively, with the contact blades36 and 37, Y

First and second contacts 44 Yand 45 are carried on the outboard ends ofthe rst and second resilient contact blades 36 and 37, respectively, formutual cooperation; and these contact blades extend longitudinally ofthe frame 32 and of the control housing 13.

The frame 32 has .a threaded aperture 47 in which is received anadjusting screw 48 which extends externally and transversely of thehousing 13 to receive a control knob 49 having an index mark 50cooperating with' indicia 51 on the upper side of the control housing13. This control knob 49 and adjusting screw 48 rotate to move aninsulator pin 52 longitudinally of the axis thereof, which is transverseto the axis 53 of the control housing 13. The lower end of the insulatorpin 52 contains a hardened bearing insert 64 for bearing against thecontact blade 36. A stop lug 54 is pressed upwardly out of the metal ofthe frame 32 and this cooperates with a stud 55 fastened to the side ofthe adjusting screw 43 to limit the screw to approximately two hundredseventy degrees rotation. The insulator pin 52 bears against the upperside of the first contact blade 36 to resist the natural resiliencythereof which tends to move this blade 36 toward the frame long leg 33.Rotation of the knob 49 therefore raises and lowers the position of thefirst contact 44.

The second contact blade 37 is shown in Figure 6 as it appears beforeassembly into the probe thermostat 11, and the first end 56 thereof isthat which is mounted in the stack 35 with the contact 45 mounted on theoutboard end 57 thereof. A tongue or hinge aperture 58 is formed nearthe first end 56 and merges with a rivet aperture 59 formed to receivethe rivet 38. The metal that formerly was in the plane of the contactspring 37 and was in the tongue aperture 58 is bent generally at rightangles to form a tongue 60. This tongue may have reinforcing ribs 62formed therein. The end of the tongue 60 has a socket depression 63.This socket depression 63 is along the axis 53 of the control housing 13and along the axis of the probe 12. Hinge portions 61 lie on either sideof the tongue aperture 58 and integrally join the first and outboardends 56 and 57 of the contact blade 37.

This probe 12 is mounted on the frame 32 specifically on the short leg34. The probe includes rst and second members of different thermalcoeficients of expansion and preferably includes a heavily walledaluminum tube 66 as an expansion member and an inner non-expansion orlow expansion member 67. The inboard end of the aluminum tube 66 isthreaded to be received in a threaded aperture 71 in the frame short leg34 and is held in place by a lock nut 65. This inner member 67 is in theform of a rod or tube and may be a vitreous or porcelain-like or ceramicmember. In the preferred embodiment this inner member 67 is tubular toreceive hardened bearing members 68 and 69 at each end there* of. Thebearing member 68 bears against the closed outer end 70 of the aluminumtube 66, and the bearing member 69 bears against the socket depression63. The spring 'blade 37 has a natural resiliency ten-ding to urge itdownwardly or away from the frame long leg 33. This urging will cause abending primarily at the hinge portions 61. With the electrical switchmechanism 30 completely assembled and the probe thermostat 11 at roomtemperature, the inner member 67 is under compression caused by thenatural resiliency of contact blade 37 acting through the tongue 60.Therefore, this nonexpansion member 67 acts to hold the second contact45 in a given or determinable position. With this contact 45 so held,adjustment of the control knob 49 will permit the first Contact 44 to bemoved into and out of engagement with the contact 45.

The control housing 13 may also enclose a neon glow lamp 72 to beobserved through a transparent window 73 in the upper part of thehousing, and the circuit diagram of Figure 7 shows that this glow lamp72 may be connected in series with a current limiting resistor 74yacross the female connectors 15 and 16. The female connector 15 isconnected to a first wire 75 of a twowire flexible electrical supplycord 76, and the second wire 77 of this cord 76 is connected through theelectrical switch mechanism 30 to the female connector 16.

In operation, the probe thermostat 11 may be plugged in to theelectrical heating appliance 1S with the male prongs 24 and 25 receivedwithin the female connectors 15 and 16. When so positioned, the probe 12will be closely received within the cylindrical aperture 20.

The control knob 49 may be rotated to the desired operating temperatureand, when the probe 12 is at normal room temperature, this will meanthat the contacts 44 and 45 are in engagement. If the control knob 49 isrotated to the off position shown in Figure 2, this will move theinsulator pin 52 upwardly as shown in Figure 3 so as to raise contact 44and open circuit contacts 44 and 45 of the switch mechanism 30. When theplug 73 is connected to an electrical outlet and the control knob 49 isrotated to an operating temperature condition, the switch 30 will beclosed and the circuit will be completed through the heating element 21to heat the frying pan 18. This heating will heat the base 19 and alsoheat the probe 12. This heat will be received by both of the members 66and 67; but with the member 66 being of aluminum it will be a highexpansion member relative to the inner member 67. This will move theouter end 70 of the tube 66 to the right as viewed in Figure 3; hence,the entire inner member 67 will move to the right because of the springurging of the natural resiliency of the contact blade 37. This actsthrough the tongue 60 and thus the spring contact blade 37 will movedownwardly to open circuit the contacts 44 and 45 at the pre-determinedtemperature setting of the control knob 49. When the contacts 44 and 45are closed, this would complete a circuit through the glow lamp 72, butthis lamp will be extinguished upon opening the contacts 44 and 45. Suchopening also deenergizes the heating element 21 and thus the entirefrying pan will cool. Such cooling will cause contraction of thealuminum tube 66 to urge the inner member 67 to the right to close thecontacts 44 and 45. .Again a heating portion of the cycle will beobtained to heat the base 19 and the aluminum tube 66 to again open thecontacts 44 and 45 by moving the contact 45 away from contact 44 to thusstart the cooling portion of the cycle. This alternating heating andcooling will continue as long as electrical energy is supplied throughthe electrical cord 76.

The control knob 49 may be rotated to any desired position which may betwo hundred to ve hundred degrees Fahrenheit, for example, and withelectrical energy supplied to the probe thermostat 11, the temperatureof the appliance 1S will adjust itself to the new operating temperaturecondition and then cycle through heating and cooling portions as recitedabove.

The probe thermostat 11 of the present invention has been found to beextremely long lived and reliable throughout its life as well asmaintaining accurate temperature calibration throughout its life, withthis ternperature calibration being as obtained by the index mark 50cooperating with the indicia 51.

In life tests under controlled identical conditions and identicalheating appliances the present probe thermostat 11 has been found todrop in calibration by only seventeen degrees after over eight hundredhours continuous use, whereas the prior art unit employing weldedconstruction dropped fifty-nine degrees in temperature calibration. Thishigh consistency in temperature calibration throughout life is due tothe heavily walled aluminum outer member 66 and the relatively rigidceramic or porcelain inner member 67 which is stressed slightly incompression and which is very stable in dimensional characteristicsthroughout life regardless of being subjected for long periods of timeto high temperatures, this coupled with the hardened bearing inserts 68and 69 which assure closely controlled length dimensions of such innermember 67 throughout life. The probe thermostat 11 of the presentinvention has also been found to be highly reliable throughout lifedespite repeated rotation of the control knob, that is, adjustment ofthe operating temperature. The contact blades 36 and 37 are preferablymade of heavy gauge stainless steel which successfully resists continuedheating and which do not lose their spring tension. The particularconstruction of the probe thermostat 11, wherein the aluminum tube 66 isfar removed from the contacts 44 and 4S and from the spring blades 36and 37, and wherein there is not good heat transfer from the aluminumtube 66 to the frame 32, permits these spring blades 36 and 37 to becool relative to the probe tube 66. This permits long life with thesecontact blades retaining their spring tension and not changing theircalibrated position throughout life.

Still further, the inner member 67, being of a porcelain or ceramicmaterial, transmits very little heat so that the tongue 60 does notbecome heated appreciably; and therefore, heat is not received by thecontact blade 37 by this means. Also, the tongue 60 is reenforced by theribs 62 so that the angular relationship of the tongue 60 and contactblade 37 remain constant throughout life with this contact blade 37bending primarily at the hinge portions 61 which do not become heated toaffect their reliability or elasticity.

In prior art thermostats the continued rotation of the control knob 49for adjustment purposes changes the calibration of the thermostatthroughout life. This has been found to be especially true where thethermostat has been adjusted for and is operating at a high temperaturesetting. lf then the control knob is adjusted to a low temperaturesetting, this heavily stresses the various parts of the thermostat, andafter only a very few such temperature calibration changes the prior artthermostats have been found to be incapable of opening the contacts; andtherefore, there is no off position at the lower end of the temperaturecalibration range. With the construction of the present invention afterstemrotation tests, it has been found that the temperature calibrationchanged only four degrees after five hundred total cycles of stemrotation from high to low back to high temperature settings, with theappliance and probe thermostat operating at the high temperature. Theprior art form of thermostat, operating with an identical appliance andunder identical conditions, increased in temperature calibration byforty-eight degrees for this same five hundred cycles of stem rotationtests; and additionally the thermostat was so damaged by this sternrotation test that it was incapable of open circuiting the contacts inthe lowest temperature position and thus there was no electrical oft`condition capable of the thermostat after such test. The particularconstruction of the thermostatv of the present invention, which permitsthis high degree of constancy in temperature calibration despiterepeated adjusting screw rotation, is primarily caused by the shortsturdy construction of the adjusting screw 48 and the insulator pin 52,together with the hardened bearing insert 64 to accurately position thecontact blade 36. Also, the constancy of length dimension of the innermember 67, which is not stressed in tension but only stressed slightlyin compression, and the fact that the contact blade 37 is notappreciably heated, all combine to give extraordinary constancy oftemperature calibration even with cycling of position of the adjustingscrew 48.

It will be noted that the entire working mechanism of the thermostat ilis carried on the frame 32 so that there is no dependence on the housing13 being in place and no dependence on the position of this controlhousing 13 to maintain calibration of the thermostat 11.

The use of the inner non-expansion member 67 as being a member stressedin compression is a safety feature because any breakage of this unit dueto abuse or extremely rough handling results in a device which failssafe This is because the contacts 44 and 45 would be open circuited insuch an event and thus the appliance 18 would not become overheated.Also, any fracture of the low expansion member 67 causes either a slightdrop in temperature calibration or causes the thermostat to fail in anelectrical open condition which is again a safety feature. This is incontradistinction to the prior art which relied on welded linkage and,if any of the welds fail, the device could fail in an electrical closedcondition to thus provide an extremely unsafe condition.

Although this invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been made only by way of exampleand that numerous changes in the details of construction and thecombination and arrangement of parts may be resorted to withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

What is claimed is:

l. A thermostatic switch, comprising, a control housing having an axis,a frame within said housing, first and second metallic spring contactblades mounted at the first ends thereof -on said frame in mutuallyinsulated relationship and insulated from the exterior of said housingand each extending longitudinally in said housing, first and secondcontacts carried for mutual cooperation on the outboard ends of saidfirst and second contact blades, respectively, means urging apart saidfirst and second contacts, an adjustable screw carried on said frame anddisposed to adjustably relatively position said first and -secondcontacts, an L-shaped tongue fixedly attached to said second contactblade and with said tongue extending generally perpendicularlyto saidsecond contact blade and having a socket depression, a surface definingan aperture between said first end and said tongue in said secondcontact blade with said aperture being centrally located laterally onsaid second contact blade, first and second hinge portions of saidsecond Contact blade on either side of said tongue aperture and joiningsaid first and outboard ends of said second contact blade, a heatexpansible tube carried by said frame and extending longitudinally ofand external of said housing, said tube having a closed outer end, asubstantially non-heatexpansible member within said tube abutting theclosed outer end thereof and abutting said socket depression on saidL-shaped tongue to establish a determinable position of said secondContact, whereby as said tube is heated it longitudinally expands andpermits said urging means to bend said second contact blade at saidhinge portions to move said second contact away from said first contact.

2. A thermostatic switch, comprising, a control housing having an axis,a frame within said housing, first and second metallic spring contactblades mounted at the first ends thereof on said frame in mutuallyinsulated relationship and insulated from the exterior of said housingand each extending longitudinally in said housing, first and secondcontacts carried for mutual cooperation on the outboard ends of saidfirst and second contact blades, respectively, means urging apart saidfirst and second contacts, an adjustable screw carried on said frame anddisposed to adjustably relatively position said first and secondcontacts, an L-shaped tongue fixedly connected near said first end ofsaid second contact blade and with said tongue extending generallyperpendicular to said second contact blade and having a socketdepression, a surface defining a hinge aperture in said second contactblade between said first end and said tongue thereof with said hingeaperture being centrally located laterally on said second contact blade,first and second hinge portions of said second contact blade on eitherside of said hinge aperture and joining said first and outboard ends ofsaid second contact blade, a heat expansible tube carried by said frameand extending longitudinally of and external of said housing, said tubehaving a closed outer end, a

lasrseftso' substantially non-heat-expansible member within said tubeabutting the closed outer end thereof and abutting said socketdepression on said L-shaped tongue to establish a determinable positionof said second contact, whereby as said tube is heated it longitudinallyexpands and permits said urging means to bend said second contact bladeat said hinge portions to move said second contact away from said firstcontact.

3. A thermostatic switch, comprising, a metallic frame with L-shapedlong and short legs with said long leg having an axis, first and secondspring contact blades with at least one mounted near the junction ofsaid long and short legs in mutually insulated relationship and eachextending longitudinally relative to said frame, iirst and secondcontacts carried for mutual cooperation on the outboard ends of saidfirst and second Contact blades, respectively, means urging apart saidrst and second contacts, an adjustable screw carried on said frame anddisposed to adjustably relatively position said rst and second contacts,rst and second female electrical connectors disposed axially parallel tosaid frame with said first connector being electrically connectedthrough said contacts to an electrical supply cord, an L-shaped tonguefixed to said second blade and extending generally perpendicularly awayfrom said frame and having a socket, a tube aperture in said frame shortleg, a heat expansible tube carried in said tube aperture and extendinglongitudinally of said frame, said tube having a closed outer end, asubstantially non-heat-expansible member within said tube abutting theclosed outer end thereof and abutting said socket on said L-shapedtongue to establish a determinable position of said second contact,whereby as said tube is heated it longitudinally expands to permit saidurging means to relatively move apart said lirst and second contacts.

4. A thermostatic switch, comprising, a control housing having an axis,a metallic frame within said housing, said frame having E -shaped longand short legs, means for holding said frame Within said housing withsaid long leg longitudinally disposed, first and second spring contactblades with one mounted near the junction of said long and short legs inmutually insulated relationship and insulated from the exterior of saidhousing and each extending longitudinally in said housing, first andsecond contacts carried for mutual cooperation on the outboard ends ofsaid iirst and second Contact blades, respectively, means urging apartsaid irst and second contacts, an adjustable screw carried on said frameand disposed to adjustably relatively position said first and secondcontacts, lirst and second female electrical connectors disposed axiallyparallel in said housing and electrically connected through saidcontacts to a two wire supply cord, an L-shaped tongue fixed to saidsecond blade and extending generally perpendicularly away from saidframe and having a socket depression, a tube aperture in said frameshort leg, a heat expansible tube xedly carried in said aperture andextending longitudinally of and externally of said housing, said tubehaving a closed outer end, a substantially non-heat-expansible memberwithin said tube abutting the closed outer end thereof and abutting saidsocket depression on said L-shaped tongue to establish a determinableposition of said second contact, whereby as said tube is heated itlongitudinally expands and permits said urging means to relatively moveapart said iirst and second contacts.

5. A thermostatic switch, comprising, a control housing of electricalinsulating material and having an axis, a metallic frame within saidhousing, said frame having L.shaped long and short legs, means forholding said frame within said housing with said long leg longitudinallydisposed, a stack mounted on said long leg near the junction with saidshort leg, iirst and second spring contact blades with at least onethereof mounted in said stack and in mutually insulated relationship andeach extending longitudinally in said housing, lirst and second contactscarried for mutual cooperation on the outboard ends of said rst andsecond contact blades, respectively, means urging apart said first andsecond contact blades, an adjustable screw carried on said frame anddisposed to abut one of said contact blades to adjustably relativelyposition said first and second contacts, first and second femaleelectrical connectors disposed axially parallel in said housing andelectrically connected through said contacts to a two wire supply cord,an L-shaped tongue iixed to said second blade and extending generallyperpendicularly away from said frame long leg and having a socketdepression, a tube aperture in said frame short leg, a heat expansibletube xedly carried in said aperture and extending longitudinally of andexternal of said housing, said tube having a closed outer end, asubstantially nonheat-expansible rod within said tube abutting theclosed outer end thereof and abutting said socket depression on saidL-shaped tongue to hold said second contact in a determinable positionrelative to said first contact, whereby as said tube is heated itlongitudinally expands and permits said urging means to move said secondcontact out of engagement with said rst contact.

6. A thermostatic switch, comprising, a control housing of electricalinsulating material and having an axis, a metallic l'rame within saidhousing, said trarne being L-shaped with long and short legs, means forholding said frame within said housing with said long leg longitudinallydisposed, a stack mounted on said long leg near the junction with saidshort leg, rst and second spring contact blades mounted in said stack inmutually insulated relationship and each extending longitudinally insaid housing, first and second Vcontacts carried for mutual cooperationon the outboard ends of said tirst and second contact blades,respectively, said rst blade being lbetween said frame and said secondcontact blade and having natural resiliency urging same toward saidframe, said second blade having natural resiliency urging same away fromsaid first blade, an adjustable screw carried on said frame and disposedto abut said lirst blade to adjustably position said rst Contactrelative to said frame, first and second female electrical connectorsdisposed axially parallel in said housing and electrically connectedthrough said contacts to a two wire supply cord, an L-shaped tonguefixed to said second blade and extending generally perpendicularly awayfrom said frame and having a socket depression, a threaded aperture insaid frame short leg, a heat expansible tube threaded in said apertureand extending longitudinally of and external of said housing, said tubehaving a closed outer end, a substantially nonheat-expansible rodinsulatedly carried within said tube and abutting the closed outer endthereof and abutting said socket depression on said L-shaped tongue tourge said second contact toward engagement with said rst contact,whereby as said tube is heated it longitudinally expands and permits thenatural resiliency of said second blade to move said second contact outof engagement with said lirst contact.

7. A thermostatic switch, comprising, a control housing of electricalinsulating material and having an axis, a metallic frame within saidhousing, said frame being L-shaped with long and short legs, means forholding said frame within said housing with said long leg longitudinallydisposed, a stack mounted on said long leg near the junction with saidshort leg, iirst and second spring contact blades mounted in said stackand each separately insulated from said frame and each extendinglongitudinally in said housing, rst and second contacts carried formutual cooperation on the outboard ends of said rst and second contactblades, respectively, said first blade being between said frame and saidsecond contact blade and having natural resiliency urging same towardsaid frame, said second blade having natural resiliency urging same awayfrom said iirSt blade, an adjustable screw carried on said frameperpendicular to said am's and disposed to abut said first blade toadjustably position said first contact away from said frame against theurging of said rst contact blade, a control knob on said screw, rst andsecond female electrical connectors disposed axially parallel in saidhousing, means for electrically connecting said irst` female connectorthrough said contacts to one wire of a two wire supply cord and meansfor connecting said second female connector to the other wire of saidsupply cord, an L-shaped tongue fixed to said second blade and extendinggenerally perpendicularly away from said frame and having a socketdepression facing said short leg of said frame, a threaded aperture insaid frame short leg aligned with said socket depression, a heatexpansible tube threaded in said aperture and extending longitudinallyof and external of said housing, said tube having a closed outer end, asubstantially non-heat-expansible ceramic rod within said tube abuttingthe closed outer end thereof 10 and abutting said socket depression onsaid L-shaped tongue to urge said second contact toward engagement withsaid first contact, whereby as said tube is heated it longitudinallyexpands and permits the natural resiliency of said second blade to movesaid second contact out of engagement with said first contact.

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